fossils (the restless earth)

1 Fossils: From Natural Curiosities to Scientific Treasures 7 2 The Tortuous Road to Fossilhood 22 3 So Many Fossils, So Little Time 38 4 Marking Turning Points in Evolution 50 5 Findi

Fossils

THE REsTlEss EARTH

Earthquakes and Volcanoes

Fossils Layers of the Earth Mountains and Valleys Rivers, Lakes, and Oceans Rocks and Minerals

Fossils

gary raham

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1 Fossils: From Natural Curiosities

to Scientific Treasures 7

2 The Tortuous Road to Fossilhood 22

3 So Many Fossils, So Little Time 38

4 Marking Turning Points in Evolution 50

5 Finding and Excavating Fossils 66

6 Fossils in the Human Family 78

Bibliography 97 Further Reading 101 Picture Credits 104

About the Author 111

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The tyrannosaur hurt The breeze off the great water relieved the sun’s heat, but her leg and side still ached where blood oozed from the gashes created by Three Horn’s nose spike She blinked her eyes, but the tattered fern fronds nearby failed to focus prop- erly Suddenly, the sky tilted alarmingly and one side of her body struck the cool earth She found that she could not move The familiar forest odors of pine resins and molding leaf litter settled about her as the world became silent and faded to black.

MORE THAN 65 MILLION YEARS PASSED

In the year 1992, a man named Charles Fickle took a walk with his dog through a half-built subdivision in Littleton, Colorado He (or maybe his dog) found a large, rock-hard bone

sticking out of the ground and suspected that it might be a sil—the (usually) mineralized remains of a once-living creature

fos-Fickle alerted the Denver Museum of Nature and Science In

response, the museum sent paleontologists—scientists who

spe-cialize in studying the remains of ancient plants and animals—to the site They unearthed the entire right leg, ten teeth, a shoulder

1

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Fossils:

FROM NATURAL CURIOSITIES TO SCIENTIFIC TREASURES

blade, and a tail vertebra, all belonging to the meat-eating

dino-saur Tyrannodino-saurus rex Fickle’s dog did not get to chew on the

bones, but Fickle got to chew on an unsettling thought: The world was once a vastly different place from what it is today

What other fossil mysteries lie buried in the Earth awaiting discovery? Can these fragments of former lives serve as a lens through which prehistoric worlds come into focus again?

Ancient Romans would have called anything dug up from the

ground a fossilium That word became fossile in French, which

came to refer, with a similar meaning, to everything from a miner’s gold nugget to a burrowing crab People often puzzled over peculiar “formed stones” that looked like giant or misshapen versions of familiar—or not so familiar—shells, bones, and ani-mals Naturalists eventually reserved the word fossil to describe such lifelike stones Fossils are clues to old mysteries that demand explanations: When did this creature live? What did it look like when it lived? Why did it become extinct?

Fossils, Myths, and Monsters

Citizens of the classic civilizations of ancient Greece and Rome often answered such questions with myths and stories The Greek city-states that nestled around the northeastern shores of the Mediterranean Sea roughly 2,500 years ago produced enough wealth to allow some of their citizens the time and means to travel within Greece, to Mediterranean islands, and to more dis-tant lands where they encountered the fossilized bones of giant creatures

Sometimes these fossils resembled smaller living species; but, sometimes, they appeared to be quite different After Greeks had first seen living African elephants around 300 b.c., they correctly identified the bones of ice-age mastodons as oversized versions of elephants Before that time, the hole in elephant skulls where the trunk attaches may have looked like a giant eye socket and given rise to legends about monstrous, one-eyed men called Cyclopes

Often, oversized bones were interpreted as the remains of heroes from Greek mythology and placed in temples or reburied with

of nature, or even as deliberate creations of a devil intent on confusing mere mortals.

tongue stones and the insights oF nicolaus steno

In the autumn of 1666, fishermen came upon a huge great white shark washed ashore on an Italian coastline Perhaps because great whites can become “man eaters,” they lashed the still-thrashing animal to a tree and killed it The Grand Duke Ferdinando II in Florence, Italy, soon learned about the fisher-men’s adventure and ordered them to deliver the carcass to his palace By that time, the shark’s body was a bit ripe with decay, but the fishermen cut off the animal’s head, loaded it on a cart, and sent it to the duke

The duke respected knowledge and admired skilled and ligent people In fact, he had sheltered an astronomer named Galileo Galilei, who supported the then-radical idea proposed by Copernicus that the Earth orbited the sun and not vice versa after observing the satellites of planets with his newly invented tele-

intel-scope In 1666, according to Alan Cutler, author of The Seashell

on the Mountaintop, “Ferdinando’s court was home to a scientific

academy founded by several of Galileo’s former pupils, mined to keep his spirit alive.”

deter-Even though the duke entertained many learned men at his academy, he chose Nicolaus Steno (1638–1686) to dissect the great white shark Nicolaus, though only 28, had already made a

(continued on page 12)

From as early as 675 B.C., Greek travelers told tales of strange, sized beasts called griffins that possessed huge, hooked beaks not unlike those of an eagle They supposedly lived in the rugged desert country of central Asia near the Altai Mountains of what is now

lion-Mongolia Adrienne Mayor, in her book The First Fossil Hunters, relates

that Aelian, a learned compiler of facts and knowledge concerning natural history in the early A.D third century, wrote, “The Bactrians say that griffins guard the gold of those parts, which they dig up and weave into their nests .”

Griffins, Mayor contends, represent the first attempt to stand and reconstruct dinosaur fossils

under-Griffins: Mythological Beasts or Dinosaurs?

Griffins (left) are

considered to be one

of the first attempts

by humans to understand dinosaur fossils—such as those

of the Protoceratops

(opposite page), the

fossils of which were mistaken to be that

of the mythological griffin

Fossils 11

The American paleontologist Roy Chapman Andrews (1884–1960) visited Mongolia in 1922 and found that the bony remains of dino-saurs were “strewn over the surface almost as thickly as stones.” He and his team recovered more than 100 nearly complete skeletons

of Protoceratops and Psittacosaurus, both of which display sive, beaked heads Protoceratops has neck frills on a lion-sized

mas-body Many white skeletons, partially eroded from the sides of red

sandstone cliffs, stood out clearly in upright positions like eternal

guards Other skeletons lay on or near clutches of birdlike eggs, or close to the remains of young dinosaurs

Flecks and chunks of gold erode from nearby mountains and sometimes wash into fossil-bearing sediments Russian archaeolo-

gists once found the skeleton of a Bronze Age miner in the area

whose leather bag still contained several gold nuggets It is no wonder that ancient travelers who might have found this victim of the desert’s heat and fierce storms might also believe that he was killed by living examples of the fierce-looking fossil creatures lying all around him

reputation for himself as a master anatomist—a person skilled at dissection and observation By this time, he had already discovered the duct that carries saliva from the parotid gland to the mouth

in humans—something that generations of physicians before him had failed to notice

A crowd gathered to watch Steno begin his dissection The sight of the dead shark with bulging eyes and jaws large enough

to consume a person must have presented an amazing spectacle

Each jaw held 13 rows of teeth; the inner ones were soft and half buried in the animal’s gums Although the fishermen had cut some of the shark’s teeth out for souvenirs, many of the teeth remained; the largest ones were perhaps 3 inches (7.6 centi-meters) long Steno realized immediately that the shark’s teeth closely resembled objects known as “tongue stones.” The mys-terious tongue-shaped rocks were sold locally for their supposed medical and magical powers; since their origins were unknown, people thought that they grew inside the rocks in which they were found Steno realized that the shark’s teeth resembled tongue stones because they were one and the same thing—“as alike as one egg resembles another.” Yet somehow the tongue stones had

petrified, or turned to stone.

For many years naturalists and travelers explained away things like seashells on mountaintops The Earth has “plastic forces” that just makes weird things, they said, or maybe the rain causes fossils to sprout like plants But Steno and other careful observers saw that finding the assemblages of shells, shark teeth, and other marine creatures all together only made logical sense if these creatures had once been alive and living

in an ocean—even if that ancient ocean bed had since risen to mountain heights

Steno’s contemporary in England, Robert Hooke (1635–

1703), came to much the same conclusion a year later while looking at fossil seashells and petrified wood through his newly

invented microscope In his book Micrographia, which was

writ-ten for the scientists of the recently formed Royal Society and

(continued from page 9)

Fossils 13

dedicated to King Charles II of England, he said, “That ers of these Shells, according to the nature of the substances adjacent to them, have, by a long continuance in that posture, The Grand Duke Ferdinando II in Florence, Italy chose Nicolaus

oth-Steno (above) to dissect a great white shark.

been petrify’d and turn’d into the nature of stone, just as I even now observ’d several sorts of wood to be.”

The insights of Steno and Hooke—that fossils represented the remains of once-living creatures—began a revolutionary change

in the way people viewed the world If the forces of nature could transform ocean beds (and the creatures they contained) into stone while piling them up into mountains, then the Earth must have a history—a very long history This concept ran contrary to orthodox Christian convictions of the time: that God had created nature all at once and pretty much “as is.”

Steno’s study of shark teeth led to many geological tions that he summarized in an essay for his patron, Grand Duke Ferdinando “In various places,” Steno wrote, “I have seen that the earth is composed of layers superimposed on each other at an angle to the horizon.” Steno realized that, like the layers of pearl that form around a sand grain, those layers implied a history The oldest layers must be on the bottom of the pile and the younger layers on top This last statement summarizes what geologists

observa-now call Steno’s principle of superposition.

“Water is the source of sediments,” said Steno, and when water fills a container, whether that container is a glass or a vast basin, gravity ensures that the surface of the water is parallel to the horizon As rocks and finer particles settle out, they will also come to lie horizontally Steno’s second principle, then, is the

principle of original horizontality If rock layers are tilted,

that tilting must have happened after the sediments originally formed

These were simple ideas, but not obvious ones They made people realize that a fossil or any natural object contains clues

to its own history Steno’s insights opened a vast new tive on living things The Earth transformed from a static stage for human activities into a restless, dynamic planet that not only changed the kinds of life it supported, but was changed by that life in turn Fossils, the relics of ancient life, became the key to understanding Earth’s long and exciting story, although it took time before everyone recognized their importance

perspec-Fossils 15

sea Monsters, Fossil hunters, and the Mystery oF extinction

When 10-year-old Mary Anning’s (1799–1847) father died in

1810 while hunting for fossils on the slippery cliffs of Lyme Regis

on the east coast of Great Britain, she not only had to deal with the tragedy, but also find a way to help her poor family survive

Deborah Cadbury, in her book Terrible Lizard, says that the

fam-ily had depended on her father’s work as a carpenter to provide money, although they did make a few shillings selling “natu-ral curiosities,” like fossils, to tourists One day, Mary found a beautiful snakestone—a fossil that today would be called an

ammonite (Its spiral shell reminded people of a coiled snake.)

Mary ran through town showing off her discovery A rich woman tourist offered her a crown for the find—a coin that could buy a week’s worth of food Mary realized that hunting for fossils along the rocky coastline could be both fun and profitable

The next year, her brother Joseph found a huge, four-foot long skull eroding out of the cliff The skull sported wicked-looking teeth like a crocodile, but had a pointed snout and large, round eye sockets almost like those of a bird After a fierce storm the following year, Mary found the rest of the creature’s body All the bones were attached, although some were crushed When towns-people helped her remove the slab of rock on which the fossil rested, they found that the creature measured 17 feet (5 meters)

long She sold that fossil (later named Ichthyosaurus, or “fish

lizard”) for enough money to feed her family for six months

Perhaps more importantly, she also attracted the attention of Reverend William Buckland (1784–1856), a student of the new

science called geology.

Buckland was a rich gentleman, but he did not mind wading

in the ocean or climbing cliffs with Mary He said once that rocks

“stared me in the face, they wooed me and caressed me, saying at every turn, Pray, Pray, be a geologist.” In fact, he thought that geol-ogy was a “master science through which [he] could under-stand the signature of God.” His position at Oxford University helped make Mary’s discovery, and those of other English fossil

enthusiasts, more widely known The man to impress in those days—Georges Cuvier (1769–1832)—was an ambitious French anatomist who lived in Paris and had a rich collection of fossils

at his disposal in the Museum of Natural History there He would later be called the father of modern paleontology

After dissecting many animals in the course of his cal studies, Cuvier was convinced that fundamental laws govern the construction of animal anatomy just as the laws of force and motion discovered by Isaac Newton determine the motions

anatomi-of stars and planets In other words, predators will always have teeth and claws designed for grappling with prey and strong, agile bodies to pursue them Swimming animals will possess fins and streamlined bodies, even those creatures that look nothing like those living today—such as Mary Anning’s bizarre fish lizard

Cuvier and other fossil hunters of the time were aware of something else: Fossils did not appear randomly among the rocks Older rocks contained a different collection of fossils than younger rocks and fossil creatures became progressively different

in deeper (and thus older) layers of rock Many of the fossils Cuvier found in what were called Tertiary rocks near Paris, for example, consisted of large mammals that often resembled liv-ing forms—much like the bones familiar to the ancient Greeks

The Ichthyosaurus lived through two great extinctions but

disappeared before the dinosaurs 66 million years ago These

shale fossils show an Ichthyosaurus mother with an infant and five

unborn babies

built new ones—a theory called catastrophism.

In fact, all of Cuvier’s options are correct to some degree

Every now and then, animals once thought to be extinct are found in some remote location In 1938, a species of fish thought

to be extinct for 70 million years, the coelacanth, was found in

an African fish market; a live specimen was later captured off the coast of the Comoro Islands Such creatures are often referred to

as living fossils.

In 1859, a scientist named Charles Darwin (1809–1882)

convincingly showed in his book The Origin of Species that

liv-ing thliv-ings do change over time—or evolve—through a process he

called natural selection Darwin showed how living things with

slight advantages can reproduce more effectively than other ing things, and so their genes will be passed on Over time—huge

liv-stretches of geological time that were becoming more and more

evident to paleontologists—small differences become very large and noticeable

tropical reptiles—except they were much bigger Eventually, more

1 ½ inches ?” If perfectly created species were truly eternal, where were the living examples of these bizarre and monstrous beasts?

Jefferson thought he knew the answer: “In the present interior

of our continent,” he said, “there is surely space and range enough for elephants and lions.”

In 1803, when he had been president of the United States for two years, Jefferson saw an opportunity to fund the exploration

of western North America Using $2,500 from Congress and some

of his own money, he directed Captain Meriwether Lewis to find a trade route from the Missouri River to the mouth of the Columbia

Lewis enlisted his friend William Clark to share the command While they prepared to get underway, Jefferson negotiated the Louisiana Purchase, giving the United States title to all the land between the Mississippi and the “Stony Mountains.” The expedition produced an amazing record of the natural history and Native American cultures

en route and laid the foundations of the united states geological

survey.

Of those fossil specimens sent to him by Stewart in 1796, one

species bears Jefferson’s name: Megalonyx Jeffersoni, or “great claw.”

It was not a huge lion as he had first thought, but a ground sloth the size of an elephant Sadly, it was no longer living in the unexplored western wilderness of America at the time of the Lewis and Clark expedition, but had vanished with the continental glaciers 10,000 years before

Fossils 1

complete skeletal discoveries showed that Mary had found the thumb spike of a giant reptile-like creature that was later named

Iguanodon because of its similarities with living iguanas Some

twenty years later, paleontologist Richard Owen (1804–1892)

coined the term dinosaur to describe a diverse group of mostly

huge vertebrates (now called archosaurs) with many reptile-like characteristics Dinosaur literally means “terrible” or “fearfully great” lizard, although dinosaurs are not lizards in the modern definition of the term

Although first discovered in England (if we exclude the coveries of ancient cultures), travelers and explorers in North America soon turned up new dinosaur finds American scientist Joseph Leidy (1823–1891) at the University of Pennsylvania and Swarthmore College described early dinosaur finds starting in the

dis-1850s and later, including the Iguanodon-like Hadrosaurus, which

was found in Haddonfield, New Jersey

Surveyors, railroad men, explorers, and scientists were ing up most new fossils in the deserts, plains, and mountainous country of the American West Such discoveries excited not only scientists but the general population, too, with visions of gigantic beasts that lived long before humans walked the Earth Mantell became obsessed with “the wreckage of former lives that had turned to stone” just beneath his feet

turn-the iMportance oF Fossils today

Scientists continue to turn up amazing fossils In 1994, the bones

of Sauroposeidon were unearthed in southeastern Oklahoma—a

dinosaur that weighed about 60 tons (54 metric tons) and stood

60 feet (18 meters) tall In 2007, scientists studying a Velociraptor

unearthed in Mongolia in 1998 (a predatory dinosaur that was

portrayed in the movie Jurassic Park) found quill knobs on its

forearm bone, a feature associated with the attachment of ers In 2007, researchers found a bee trapped in 10-to-15-mil-lion-year-old amber The bee carried the pollinaria (pollen sacs)

feath-of the earliest known orchid species In 2004, fossils discovered

on the Indonesian island of Flores indicate that at least two

spe-What kinds of questions can these and other fossil discoveries help scientists answer?

F   Fossils help us know when things happened in the history

of the Earth Scientists know that Sauroposeidon lived at a

later time than Apatosaurus, for example, because it was

found in sedimentary rocks that overlay the kind of rocks in

which Apatosaurus was found—a fact that Steno would have

appreciated

F   Fossils provide glimpses of some of the lost worlds of deep time

Finds connecting insects to plants that they had pollinated provide valuable information about when certain ecological relationships between organisms evolved

F  Fossils document the major features of evolutionary change The

discovery that a predatory dinosaur like Velociraptor possessed

feathers lends support to the theory, based on anatomical and other evidence, that birds are the direct descendants of one branch of dinosaurs (More traditional dinosaurs are now often referred to as “non-avian or non-bird dinosaurs.”)

F  Fossils provide clues to the reasons for extinctions—both normal and catastrophic kinds The kind and abundance of various

fossils change dramatically at extinction boundaries In

fact, these changes define such boundaries Fossils provide scientists with clues to understanding the mechanisms that cause major extinction events, which will provide guidelines

to prevent human behaviors that might start or accelerate catastrophic extinction events today

F   Primate fossil remains provide insights into human evolution and reveal our intimate and necessary connections to the rest of the living world The ancient human species found on Flores

stood only three feet tall Human evolution on a small,

isolated island resulted in the same kind of dwarfism strated by other animals, like mammoths, that were isolated

demon-in a similar fashion Fossil discoveries demon-indicate that at various points over the last 2 million years, several human species

Fossils 21

may have coexisted, although only Homo sapiens remains

today All the fossil evidence to date indicates that human beings have developed from preexisting species and depend

on interactions with countless other living things in order to survive

Because fossils are rare and because only some organisms are likely to become fossils, scientists must always use caution in drawing broad conclusions from studying fragmentary remains that are biased in favor of creatures with easily fossilized parts and that died under certain special conditions The next chapter will demonstrate just how “lucky” a creature has to be to become an entry in the fossil record

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A MOUNTAIN LION KILLS A YOUNG DEER TO EAT AND PROVIDE FOODfor her cubs Coyotes, ravens, and other scavengers eat their fill of the leftovers and scatter the bones Microscopic organ-isms, mostly bacteria and fungi, break down living tissue into the atoms and molecules of which they are composed These recycling processes, operating over Earth’s entire 4.6-billion-year history, have ensured that life goes on Carbon atoms that build the framework of a fat molecule in a person’s big toe may

once have nestled in muscle tissue in a T rex’s jaw An oxygen

atom from a protein molecule consumed in yesterday’s hot dog may have passed through the lungs of Cleopatra Fortunately, for anyone curious about the nature and evolution of past life

on Earth, our planet does fail to recycle everything quickly

in a straightforward way Sometimes her restless forces make fossils

e(rosion)-Worlds and d(eposition)-WorldsKirk Johnson, a paleobotanist at the Denver Museum of Nature

and Science, likes to talk about D-Worlds and E-Worlds In

2

The Tortuous Road to Fossilhood

the tortuous road to Fossilhood 23

E-Worlds like Colorado, where Johnson lives, wind and water

expose fossils through erosion Rivers rush down mountains,

carving channels that expose rock that was once mud in some D-World long ago D-Worlds, then, are low places like swamps and ocean beds where the sediments scoured from E-Worlds

pile up and sometimes bury living things Deposition rules in

D-Worlds To hide from the recycling powers of nature—such as wind, water, scavengers, and decomposers like bacteria—a living thing must enjoy a quick and long-undisturbed burial after it dies

in a D-World in order to become a fossil

Obviously, a lot of things get buried quickly and never become fossils Countless worms live and die in the mud, yet almost none

of these creatures fossilize because they do not have hard parts—

things like bones, horns, shells, and teeth Soft tissues of plants

and animals can absorb or be replaced by minerals to become the

“formed stones” that have so long intrigued people, but fossils are

not always hard and mineralized A fossil consists of the remains

or traces left behind by a living creature Some remains get served for a very long time with little alteration

pre-MuMMies, “sapsicles,” and tar pit divers

In September 1991, hikers found the head and shoulders of a man melting out of a glacier high in the Alps on Italy’s border with Austria Five other bodies had been found that year to join six that were discovered between 1952 and 1990 Most of these corpses belonged to hikers or skiers who had made bad decisions or were surprised by a sudden storm Most of them had died within the past few years or decades But the so-called “Iceman,” found in

1991, had met his end on some spring or summer day 5,300 years before The ice preserved his clothes, tools, the pollen from hornbeam blossoms that floated in the air around him, and even traces of his last meal

High mountain glaciers, like the one where the “Iceman”

was found, act like refrigerators to preserve the remains of living things Low temperatures keep bacteria and other decomposers

from performing their recycling chores Because we are living during a warm interlude between a series of ice ages or glacia-tions extending back some 2 million years, the bodies of mam-moths or other extinct creatures also become exposed as the glacier ice melts.

The word mummy may conjure images of a

bandage-wrapped body chasing some archaeologist down a passage in

an Egyptian tomb Ancient Egyptians and other peoples have,

in fact, intentionally removed internal organs and applied salt and other chemicals to inhibit decay in human bodies as part

of their religious and cultural traditions Scientists do not ally consider mummified human bodies to be fossils, but natural mummification does occur in both dry and cold climates to

usu-A 10,000-year-old, 4-foot Siberian baby mammoth carcass is examined in the Arctic city of Salekhard in July 2007—a discovery that could help us understand more about climate change

the tortuous road to Fossilhood 25

both humans and other animals Three species of giant ground sloth, for example, lived in North America 13,000 years ago The

largest species, Paramylodon, could rear up 6 feet (1.82 meters)

and weighed 3,500 pounds (1,590 kilograms) This veggie eater spent time in caves—perhaps seeking sheltered space when giving birth Individual sloths also died in caves Scientists have found complete sloth skeletons with hair, skin, nails, and soft tissues like muscles and tendons dried but still intact Some sloth skin still contains small nodules of bone called ossicles, which are common in reptiles, but only found in armadillos among today’s living mammals

Insects, lizards, small mammals, and various plants and plant

parts sometimes blunder into and get stuck in the gooey resin of

certain trees The resin, over long stretches of time, heat, and

pres-sure, becomes amber As fans of Jurassic Park movies know, amber

can preserve DNA (deoxyribonucleic acid) and other complex

organic molecules for millions of years While reconstructing vidual animals (like dinosaurs) from fragmentary bits of DNA is currently impossible, creatures trapped in amber provide a wealth

indi-of detail about the world in which they lived For example, the husband and wife scientific team of George Poinar Jr and Roberta Poinar have looked at a stingless bee trapped in amber with “gos-samer wings outstretched and perfectly preserved down to the last hair” and contemplated what its eyes saw in a Dominican Republic rainforest 40 million years ago By observing and record-ing hundreds of amber “sapsicles,” they have convincingly recre-

ated this animal’s lost world in their book The Amber Forest.

A young Columbian mammoth searching for food 20,000 years ago steps on what he thinks is solid ground only to sink

into the black goo of a tar pit—a natural pool of asphalt formed

when organic material slowly decays as it heats up underground

He struggles, but only gets stuck more deeply His cries attract dire wolves, perhaps an old saber-toothed cat, and giant birds

of prey, called teratorns Some of them get stuck in the tar as well, and what looked like a “free lunch” becomes their last meal Scientists have “read” this and many similar stories while

Amber can preserve DNA and other complex organic molecules for millions of years, providing lots of detail about the world in which the organism embedded in the amber lived Above, insects more than 38 million years old are embedded in two pieces

of amber

the tortuous road to Fossilhood 27

mining the famous La Brea tar pit traps in the center of Los Angeles, California Many insects, birds, turtles, and plant parts also found their way into the tar Quick burial and low oxygen preserved their remains intact—in this case for tens of thousands

of years

carBonization: Fossil “road Kill”

A paleontologist visiting Douglas Pass, Colorado, high in the Rocky Mountains, may use her rock hammer to pop apart a thick

slab of shale and find the dark brown image of a Macginitiea

(sycamore) leaf or a fossil cranefly A musty smell reminds her

of fish and rotting vegetation on a lakeshore The leaf or the

insect has been carbonized: partially decayed, wrapped in slime,

crushed, and heated

Herbert W Meyer, a paleontologist with the U.S National Park Service, has studied this process in some detail at famous fossil beds (the site of ancient lakes) near Florissant, Colorado

The layers of shale at Florissant and Douglas Pass, as well as other locations in Utah and Wyoming, alternate with the ash from vol-canic eruptions Many of the early workers at these sites assumed that animals and plants in the lakes died and were preserved in mass dying events during volcanic eruptions, but careful studies have shown that much of this preservation happened in the peri-ods between these violent events “The thin shale layers formed slowly over many decades,” says Meyer, “whereas the layers of

volcanic ash accumulated much more rapidly.”

Images taken of Florissant fossils with a scanning electron microscope have shown that leaves and delicate insect wings

are covered with a thin layer of billions of tiny plant cells called diatoms Growing abundantly in lake water enriched with silica from volcanic ash, these diatoms and bacteria covered dead insects, fish, and leaves floating on or in the lake with a thin film that is almost like plastic wrap This film helped preserve the organisms long enough to be buried by successive layers

of silt and volcanic ash The weight of these layers ultimately

compressed and heated the remains until they became dark bon “ghosts” of the original animal or plant These carbon films often display rich patterns and fine detail, although sometimes they do look more like ancient road kill.

car-on BecoMing a rocK

Visitors to Arizona’s Petrified Forest see what most people think

of when they think of fossils: once-living things literally turned

A 50-million-year-old Eocene Age fish fossil (Priscacaraliops) was found

at the Green River Formation near Kemmerer, Wyoming

the tortuous road to Fossilhood 2

to stone Scientists rarely use the term petrification anymore

Instead, they speak of permineralization and replacement—

processes that result in the conversion of living tissues to minerals

This petrified tree trunk in Yellowstone National Park in Wyoming was transformed into its current state by volcanic activity in the area

like quartz (silicon dioxide, SiO2), calcite (calcium carbonate,

CaCO3), or pyrite (FeS2)

Imagine a huge pine tree, say an Araucarioxylon tree like

those in Arizona’s Petrified Forest, undercut and swept away in a flash flood It ultimately drifts into a wide meander of the river where sand and muck bury it Without the oxygen that bacteria and other decomposers need in order to feed, most of the tree’s wood remains intact Over a period of time, mineral-laden

Though it does not happen often, fossil hunting can become a family business Such was the case for Charles Sternberg and his sons Charlie, George, and Levi In 1908, they had just collected a

Triceratops skull in Wyoming, but were short on food and supplies

Just as Charles and Charlie got the wagon hitched and ready to take into Lusk (the nearest town, 65 miles away), George found some interesting bones sticking out of a high ridge of sandstone George and young Levi, who was fourteen at the time, decided to stay and work on the new fossil rather than risk not finding it again Charles and Charlie left to get food

George and Levi worked hard for five days until their family returned, surviving on a few old potatoes that they boiled a few at a time As they uncovered the fossil from the rock, they got more and more excited They knew they had found something special Finally, George lifted a huge slab of sandstone off the chest of the fossil animal and stared in wonder “I realized that here for the first time,

a skeleton of a dinosaur had been discovered wrapped in its own skin,” he wrote later The Sternbergs had found a dinosaur that had

The Sternbergs Find

a Dino-mummy

the tortuous road to Fossilhood 31

water filters through the tree’s muddy tomb, entering pores and even the empty spaces within individual plant cells, whose walls are made of relatively tough materials like cellulose and lignin

As water evaporates, minerals crystallize out of solution This process is called permineralization Replacement occurs when the carbon compounds making up the tough lignin and cel-lulose of the wood are later replaced by calcite, iron, or other mineral compounds

first dried out and become a natural mummy—perhaps because it died in some protected place where no predators could reach it—and then was transported to a final resting place during a flash flood that quickly buried it In this final tomb, it was petrified and preserved for more than 65 million years

George and Levi Sternberg found this mummified carcass of an

Edmontosaurus dinosaur in Niobrara County, Wyoming in 1908.

A collector can section a completely mineralized piece of wood with a rock saw and see even microscopic cellular detail

Bones have a similar porous structure that makes them perfect for permineralization The phosphate compounds that make up bone may change composition somewhat by incorporating chlorine and/or fluorine atoms rather than being completely replaced

Iron sulfides may replace calcium carbonate shells or other organic materials buried in low-oxygen, ocean-bottom mud, in a process aided by bacteria, to produce fool’s gold, or pyrite Such fossils truly look like creatures dipped in gold

casts, Molds, and other variations on the theMe

Not all fossils become exact replicas of their living model In the tree example, the sand and mud around the trunk may have hardened to stone, but all the organic material of the tree ulti-

mately decayed completely This left a hollow cylinder, or mold,

that displayed the exact shape and outer detail of the tree trunk

If this natural mold then filled with minerals or more mud before

breaking apart or eroding away, a cast of the tree would remain

behind The cast would have the shape and outer texture of the

tree but inside it could be pure agate, a form of silicon dioxide

This kind of petrified fossil is also called a pseudomorph, or

“false form.”

Shells also make good fossils because they are hard and tant to change Animals create their shells out of a form of cal-

resis-cium carbonate called aragonite Aragonite often recrystallizes

into the more stable form of calcium carbonate called calcite At other times, shells end up as casts or molds or both

Some popular fossils called trilobites are the casts, molds, and petrified remains of arthropods whose shells were composed of chitin, like modern crabs and insects A person finding a mold of

a trilobite shell can easily make a cast of the animal by pressing some clay into the mold Because trilobites and other arthropods grow larger by shedding old shells and growing new ones after their bodies take up water and expand, each animal has the

the tortuous road to Fossilhood 33

potential of leaving multiple fossils if conditions encourage silization Fossils of shed skins are an example of a special kind

fos-of fossil called a trace fossil—a remnant left behind by a living

thing

This diagram illustrates mold and cast formation, two ways that fossils develop

trace Fossils: tracKs, dung, and More

Tourists at a beach leave thousands of tracks in wet sand, but most of these footprints are washed away by the tide or covered over by other wandering tourists Now and then, the sun’s heat hardens tracks in soft mud before successive layers of sand and dirt bury and preserve them Many dinosaur tracks formed in this way along ancient seashores that bordered a seaway that once separated North America into two landmasses a hundred million years ago A number of these tracks were discovered in sandstone

in Morrison, Colorado, just west of Denver A volunteer group called the Friends of Dinosaur Ridge preserves them and shows them to many visitors every year

But sometimes, scientists do not find typical footprints

Instead, they may find footprint casts Here is how these casts form: A footprint made in mud hardens, and then wet sand fills the track depression As the water of the seaway ebbs and flows, many alternating layers of mud and sand will cover the prints Over long periods of time, the mud gets compressed to

mudstone by the weight of overlying sediments Similarly, sand

becomes transformed into a harder rock called sandstone Unless protected in some way, mudstone will erode away first leaving sandstone casts of the original footprints

Perhaps the biggest exposed dinosaur trackway in North America parallels the Purgatoire River in southern Colorado In

a remote spot that has been part of a military testing range for many years, a slab of resistant sandstone 100 yards (90 meters) long and about the width of a two-lane highway stretches away

into the distance A photo in the January 1993 issue of National

Geographic magazine captured dinosaur track expert Martin

Lockley examining prints produced by five giant sauropods

that had walked side by side Each footprint was about as wide

as the end of a telephone pole Visitors to the site can also see

the three-toed tracks of predators—perhaps Allosaurus-type

the-ropod dinosaurs Such trackways reveal secrets about the size,

the tortuous road to Fossilhood 35

Dinosaur tracks found in Picketwire Canyon in Animas County, Colorado

weight, travel speed, behavior, and ecological relationships of these extinct creatures.

Even fossilized dung (or droppings), called coprolites by

paleontologists, provide valuable clues about an animal’s diet and environment Scientists have found fragments of bone, teeth, fish scales, mollusks, wood, leaves, seeds, and even footprints (usually of micelike early mammals) in coprolites

Distinctive burrows in plant-eating dinosaur coprolites show that dung beetles, not unlike those alive today, helped recycle those wastes Other vertebrate trace fossils include eggs and the nests that sheltered them These reveal how certain dino-saurs and lizards reproduced and cared for their young Intact

Members of the Royal Saskatchewan Museum in Canada found the largest currently known fossil coprolite in 1995 in sediments from the end of the age of dinosaurs The fossil measured about 17 x

6 x 5 inches (44 x 16 x 13 centimeters) and contained chunks of bone belonging to an animal about the size of a modern cow The microscopic distribution of bone fibers and blood vessel arrangement

in the ingested bones implies that they had belonged to a juvenile dinosaur Because of the coprolite’s size (probably 2.5 quarts [2.3 liters] when fresh) and the age of the sediments, this jumbo trace

fossil probably belonged to a Tyrannosaurus rex (In fact, the tists found the coprolite while taking a break from a T rex excava- T rex excava- T rex

scien-tion just 1.25 miles [2 km] away.) Members of the science team also

recovered scattered remains of a Triceratops nearby Young Triceratops calves may well have been part of a well-balanced T rex diet T rex diet T rex

The Scoop on Some King-sized Poop

the tortuous road to Fossilhood 37

embryos in some eggs provide clues about rates of development and which eggs belong to which adults

What kind of trace fossils have humans left behind? What products of modern technology will best survive the trials and tribulations of becoming a trace fossil? Only time—and lots

of it—will tell for sure, but scientists suspect that objects like glass soda bottles and plastic forks will survive a very long time

Hopefully, a paleontologist in some future E-World will not conclude that we worshipped Barbie dolls and tiled our houses with cafeteria trays

▲ ▲ ▲

A CHINESE PROVERB SAYS, “A PICTURE’S MEANING CAN EXPRESS TENthousand words.” In 1787, geologist James Hutton (1726–1797) saw a geological feature much like the one shown on the opposite page That feature, consisting of two rock layers lying at sharp angles to each other and separated by an eroded surface, is known

in geological terms as an angular unconformity It inspired

Hutton to write two massive books (Theory of the Earth, volumes

1 & 2) He was justified in exceeding the Chinese proverb’s expectations, because the image of the rocks revealed something

profound: that the world was a very old place Fossils implied the

same thing, but not always so graphically all in one place

Nearly 200 years later, the writer John McPhee struggled, as all of us must, to understand the vast stretches of time demon-

strated by Earth’s layered topography He coined the term deep

time as a label He states in his book Basin and Range, “Numbers

do not seem to work well with regard to deep time Any number above a couple of thousand years—fifty thousand, fifty mil-lion—will with nearly equal effect awe the imagination to the point of paralysis.”

The image deserves a closer look

3

So Many Fossils,

So Little Time

so Many Fossils, so little time 3

unconForMities: gliMpses oF deep tiMe

The figure above shows a gash that exposes the rocks under the ground at the Olympic Coast in Washington State The lowest

set of layered rocks lie vertical to the ones above them When the

lower rocks were formed in some ancient sea, they must have lain horizontally—for as long as it takes to deposit sediments that thick (Remember Steno’s principle of original horizontality.)

Above is an example of angular unconformity at the Olympic Coast

in the state of Washington, with sharply dipping vertical rock layers deposited during the Miocene Era underneath horizontally bedded gravel deposited during the Pleistocene Era